1. Technical Field
The present invention relates to an optical position detecting apparatus and a display device with position detecting function including the optical position detecting apparatus.
2. Related Art
In recent years, in electronic apparatuses such as a cellular phone, a car navigation system, a personal computer, a ticket machine, and a bank terminal, a display device with position detecting function in which a touch panel is arranged on a front surface of an image generating device such as a liquid crystal device is used. In such a display device with position detecting function, a user inputs information while referring to an image displayed on the image generating device. Such a touch panel is configured as a position detecting device for detecting the position of a target object in a detection region.
As a detection system employed in such a position detecting device, a resistance film system, an ultrasonic system, a capacitance system, an optical system, and the like are known. The resistance film system is low in cost. However, the resistance film system as well as the capacitance system is low in transmittance. The ultrasonic system and the capacitance system have high response speed but are low in environmental resistance. On the other hand, the optical system has a characteristic that the environmental resistance, the transmittance, and the response speed can be respectively set high (see JP-A-2004-295644 and JP-A-2004-303172).
However, in optical position detecting apparatus described in JP-A-2004-295644 and JP-A-2004-303172, cost is high because light sources, photodetectors, and the like in a number corresponding to resolution at a position coordinate to be detected are necessary near a display screen.
Therefore, the inventor has been examining an optical position detecting apparatus including light sources for position detection 1012 provided at corner portions of a light guide plate 1060 as schematically shown in FIG. 15. In such an optical position detecting apparatus, the light sources for position detection 1012 are sequentially turned on, position detection light L2 emitted from the light guide plate 1060 is reflected on a target object Ob such as a finger, and reflected light of the position detection light L2 is detected by a photodetector 1030. In that case, since a predetermined relation holds between the intensity of the position detection light L2 emitted from the light guide plate 1060 and a distance from the light sources for position detection 1012, the position of the target object Ob such as the finger can be detected. Therefore, there is an advantage that only small numbers of light sources for position detection 1012 and photodetectors 1030 are necessary.
However, in the optical position detecting apparatus of such a system, regardless of the fact that the position detection light L2 emitted from the light sources for position detection 1012 propagate in every direction in the light guide plate 1060 while diverging, an intensity distribution in the two-dimensional directions of the position detection light L2 in a detection region 10R is controlled according to only a propagation distance of the position detection light L2. Therefore, since it is relatively difficult to form a proper intensity distribution, the position of the target object Ob cannot be accurately detected. A configuration shown in FIG. 15 is a reference example for the invention and is not a related art.